The present invention relates to a stable reagent and a method for the determination of hydrazine in aqueous solutions.
Hydrazine is a known anti-corrosive agent for steel in contact with water, for example, in steam boiler operation, in warm water circulation systems and in the shut-down preservation of vessels and the like. Depending on the temperature and pressure conditions, 0.1-300 ppm of hydrazine are added to the water. Systems under higher pressures and at higher temperatures require the smallest amounts of hydrazine. As a result of the consumption of hydrazine during operation of the steam boiler, the hydrazine contents susceptible to measurement frequently fall below 0.1 ppm. Hitherto, it has not been possible to measure these low concentrations with simple rapid tests. However, in boiler operation, the residual hydrazine content in the range below 0.1 ppm is an important parameter. This determines the time at which further hydrazine is metered in and the amount of the additionally metered hydrazine.
There is a standard method for the determination of hydrazine in water. Since its introduction in 1947, this method has formed the basis of all factory laboratory methods and rapid test methods. The reaction involved is that of hydrazine with 4-dimethylaminobenzaldehyde to produce 4-dimethyl aminobenzaldehyde azine (Bull. Soc. Chim. France, 122-123 (1947)). On the other hand, it is also known that primary and secondary aromatic amines in methanol in the presence of trichloroacetic acid give a more sensitive and deeper coloration with 4-dimethylaminocinnamaldehyde than with B 4-dimethylaminobenzaldehyde (Chemical Abstracts 55, 19616b (1960)).
However, the established and reliable method of determination of hydrazine using 4-dimethylaminobenzaldehyde has the decisive disadvantage that the solution of the 4-dimethylaminobenzaldehyde is yellow while the corresponding azine which is formed likewise has a yellow color, although this is somewhat deeper. Since the eye is able to differentiate differences in yellow intensities less well than differences in the intensities of other colors, only relatively rough grading of the hydrazine concentrations, e.g., of 0-0.1-0.25-0.5-1.0 ppm of hydrazine, has been possible hitherto with the optical test. Particular difficulties arise in the case of low hydrazine contents. These are especially difficult to assign by gradings of pale yellow shades since the blank is already yellow in color. A further problem is the optical measurement in artificial light, under which the differentiability of yellow shades is in general even poorer.
On the other hand, in contrast to 4-dimethylaminobenzaldehyde, 4-dimethylaminocinnamaldehyde has poor stability in solution. However, it is absolutely essential that a liquid ready-for-use reagent have good stability so that it will not be necessary to prepare a fresh reagent solution before each determination.